Electrolysis method and apparatus

ABSTRACT

An improved method and apparatus for the electrolysis of ionizable chemical compounds is disclosed including specifically a process for the production of chlorine and caustic soda containing low concentrations of sodium chloride by the electrolysis of brine which comprises electrolyzing brine solutions in a two compartment cell equipped with a cathode and an anode separated by permselective barrier formed by sandwiching together two or more membranes consisting essentially of a hydrolyzed copolymer of tetrafluoroethylene and a sulfonated perfluorovinyl ether having the formula 
     
         FSO.sub.2 CF.sub.2 CF.sub.2 OCF(CF.sub.3)CF.sub.2 OCF═CF.sub.2 
    
     said copolymer having an equivalent weight of from about 900 to about 1600. By the use of a multilayer membrane sandwich as the barrier the caustic current efficiency of the process is increased over that obtained when a single layer membrane is used as the barrier separating the cathode and anode.

This is a division of application Ser. No. 335,975 filed Feb. 26, 1973now U.S. Pat. No. 3,979,549.

This invention relates to a method and apparatus for the electrolysis ofaqueous solutions of ionizable chemical compounds. More particularly itrelates to the production of halogens, e.g., chlorine, and alkali metalhydroxides, e.g., sodium hydroxide, by a method which possessesadvantages over previously known methods.

BACKGROUND OF THE INVENTION

The electrolysis of aqueous solutions of ionizable chemical compounds,particularly brine solutions, in a cell equipped with an anode and acathode separated by a porous diaphragm is well known in this art. Inmost instances such cells are operated under conditions such that ionicmigration and molecular migration through the porous diaphragm occurs toa substantial degree resulting in the contamination of the cathodeliquor with undecomposed electrolyte and of the anode liquor withreaction products of the cathodic material and anodic materials.

It has been proposed to replace the porous diaphragm in such cells witha diaphragm impervious to both liquids and gasses thereby to controlboth ionic and molecular migration during electrolysis. Many patents,such as U.S. Pat. No. 2,967,807, U.S. Pat. No. 3,390,055, and FrenchPat. No. 1,510,265, disclose electrolytic cells incorporating as thediaphragm or barrier, membranes fabricated from synthetic organicion-exchange resins. Among such resins, cation exchange resins of the"Amberlite" type, sulfonated co-polymers of styrene and divinyl benzeneand others have been disclosed.

However, such resins have not been entirely satisfactory for one or moreof the following reasons:

A. The resins are not stable to strong caustic and/or concentratedacidic solutions at temperatures above about 75° centigrade.

B. The resins are effective only for relatively short periods.

C. The resins are expensive and fabrication costs are relatively high.

D. The voltage drop through the membrane becomes inordinately high asthe caustic concentration in the cathode compartment increases to aboveabout 200 gpl caustic.

E. Ion selectivity and chemical compatibility of the membrane decreasesas the caustic concentration of the catholyte liquor increases.

f. Caustic efficiency of the electrolysis decreases as the causticconcentration in the cathode compartment increases.

In application Ser. No. 212,171 of Edward H. Cook, Jr. et al, filed Dec.12, 1971, a process and apparatus is disclosed for carrying out theelectrolysis of an ionizable chemical compound, specifically sodiumchloride in a cell containing interposed between the anode and cathodethereof, a barrier composed of a single layer of a permselectivemembrane which is substantially impervious to liquids and gases, inertwith respect to the electrolyte and products of the electrolysis andwhich is composed essentially of a hydrolyzed copolymer oftetrafluoroethylene and a sulfonated perfluorovinyl ether. This cell canbe operated for extended periods without destruction of the diaphragmmaterial and produces a caustic soda product which contains a lowcontent of sodium chloride. However, it has been found that the causticcurrent efficiency has a tendency to decrease as the causticconcentration of the catholyte liquor increases above about 100 gramsper liter ("gpl").

It can be seen that prior art procedures for electrolyzing aqueoussolutions containing electrolysis wherein barriers or diaphragmscomprising an ion-exchange substance are used to separate the cathodeand anode compartments leave something to be desired.

OBJECTS OF THE INVENTION

It is thus a primary object of this invention to provide a novelelectrolysis apparatus and method which overcomes the difficultiesinherent in the prior art methods encountered in segregating thedesirable products during the electrolytic decomposition of chemicalcompounds in electrolytic cells of the barrier or diaghragm type withoutloss of the many advantages inherently connected therewith.

It is another object to devise a process utilizing as the barrier amaterial which precludes or substantially reduces both molecularmigration and undesirable ionic migration but which still permits theefficient conduction of electric current by movement of desirable ions.

It is a particular object to devise a process employing a barriermaterial for the cell which will give high purity products in high yieldwithout undue loss of electrical current and loss of product yield dueto ionic and/or molecular migration.

A further object is to devise a process employing multilayers of apermselective material as the barrier which can be operated with highcurrent efficiency for long periods without destruction of thediaphragm.

Other objects and advantages will be apparant to those skilled in thisart on consideration of this specification and the appended claims.

SUMMARY OF THE INVENTION

The objects and advantages of this invention are accomplished bycarrying out the electrolysis of an aqueous solution of an ionizablechemical compound in a cell containing, interposed between theelectrodes thereof, a barrier or diaphragm composed of at least twolayers of permselective membrane material which is substantiallyimpervious to liquids and gases, which is inert with respect to theelectrolyte and products of the electrolysis, and which is composedessentially of a hydrolyzed copolymer of tetrafluoroethylene and asulfonated perfluorovinyl ether. The diaphragm, barrier, or septumemployed in the process of this invention is comprised of at least twolayers of the permselective membrane which may be superimposed, one uponthe other, or separated by a porous material such as asbestos.

DETAILED DESCRIPTION OF THE INVENTION

In order that the invention may be more readily understood, it will bedescribed with specific reference to certain preferred embodiments, andspecifically with reference to the electrolysis of an aqueous solutionof sodium chloride whereby chlorine, caustic soda, and hydrogen areproduced. It, however, is not to be construed as limited thereto exceptas defined in the appended claims.

In accordance with a preferred mode of carrying out the invention, anaqueous solution of sodium chloride is electrolyzed in a chlor-alkalicell comprised of a vessel divided into an anode compartment containingan anode and a cathode compartment containing a cathode, thecompartments being separated by a barrier substantially impervious tofluids and gases and being composed essentially of at least two layersof a hydrolyzed co-polymer of tetrafluoroethylene and a sulfonatedperfluorovinyl ether having the formula

    FSC.sub.2 CF.sub.2 CF.sub.2 OCF(CF.sub.3)CF.sub.2 OCF═CF.sub.2

said co-polymer having an equivalent weight of from about 900 to 1600.Preferably the equivalent weight of the co-polymer is from about 1100 to1400.

Co-polymers of the character referred to above are prepared as disclosedin U.S. Pat. No. 3,282,875, by reacting, at a temperature below about110° centigrade a perfluorovinyl ether of the formula

    FSO.sub.2 CF.sub.2 CF.sub.2 OCF(CF.sub.3)CF.sub.2 OCF═CF.sub.2

with tetrafluoroethylene in an aqueous liquid phase, preferably at a pHbelow 8, and in the presence of a free radical initiator such asammonium persulfate, and subsequently hydrolyzing the acyl fluoridegroups to the free acid or salt form by conventional means.

The method and apparatus of this invention will be further describedwith reference to the attached drawing which shows a schematic view ofthe electrolytic cell, 1, comprising an anode, 2, and a cathode, 3,separated by a two layer "sandwich" of a permselective membrane barrier,4, to form an anolyte compartment, 13, and a catholyte compartment, 14.The cell, 1, has an inlet, 5, in the anode compartment, 13, for theelectrolyte, and an outlet, 6, for chlorine gas. There is also providedan inlet, 7, for charging liquid, such as a dilute aqueous caustic soda,to the cathode compartment, 14, an outlet, 8, for discharging NaOHliquor from the cathode compartment, and an outlet, 9, for hydrogen gas.

Brine is continuously circulated in the anolyte compartment 13, byintroducing brine through inlet 5 and withdrawing it through overflow,10, to the replenishing zone, 11, where the brine is replenished withsodium chloride and acidified with acid, if desired. The replenishedelectrolyte flows, via line 12, to reenter cell, 1, at inlet 5.

In the preferred embodiment, sodium chloride brine solutions containingfrom about 200 gpl to 320 gpl sodium chloride are electrolyzed in cellshaving an anode compartment and a cathode compartment separated by a twolayer sandwich of a homogeneous cation active membrane formed from acopolymer of the type described above which copolymer is substantiallyimpervious to gases and liquids, by impressing a decomposition voltageacross the electrodes disposed in each of said compartments, whilemaintaining the alkali metal hydroxide content in said cathodecompartment above about 10% by weight, and preferably from about 24 toabout 38 percent by weight, and recovering an alkali metal hydroxideproduct from said cathode compartment containing less than about onepercent by weight of sodium chloride, and chlorine from said anolytecompartment.

The present invention has the desirable advantage over many prior artelectrolysis cells and processes, in that the cell, is readily convertedfor use for the electrolysis of hydrochloric acid to produce chlorineand hydrogen, the latter being obtained substantially free fromchlorine. Thus the cells of this invention can be readily andeconomically modified for use to electrolyze either brine orhydrochloric acid, as the demand warrants. Accordingly, it is a featureof this invention that it can be efficiently operated to producechlorine at high, e.g. about 99 percent anode efficiency, and highpurity caustic soda or high purity hydrogen gas as principal products.

It is preferred when operating the cell for the electrolysis of brine,to use acidified brine as the feed to the anolyte compartment. Theaddition of hydrochloric acid to the brine feed has been found toneutralize such hydroxyl ions which migrate from the catholyte to theanolyte. The amount of acid used to acidify the brine feed can be variedover a broad range. By the addition of acid to the brine feed, the pH ofthe anolyte may be varied over a broad range also. Preferably acidadditions are made to control the pH of the anolyte and the anolyteliquor should have a pH in the range of about 1 to 5, and especiallywithin the range of about 3.0 to 4.5. The maintenance of a pH within theranges set out above in the anolyte compartment, by reducing thehydroxyl ions concentration in the anolyte, reduces the formation ofsodium chlorate in the anolyte. Thus the lower the pH in the anolyte,the less sodium chlorate is formed in the anolyte and consequently thehigher the efficiency of the cell.

In the preferred operation of the cell for the electrolysis ofhydrochloric acid, the feed to the anode compartment is an aqueoushydrochloric acid solution, desirably having an HCl content of fromabout 10% to 36% by weight and preferably of from about 15% to 25%. Thefeed to the cathode compartment may be water, although desirably it isalso an aqueous hydrochloric acid solution having an HCl content of fromabout 1% to 10% by weight with from about 1% to 5%, being preferred. Thefeed to both the anode and cathode compartments should be free of alkalimetal, or other contaminating ions, although, when a steel or othercorrodible cathode is used, alkali metal chlorides may be added to theanode compartment feed to minimize corrosion. Where such additions areused, amounts of the alkali metal chloride, e.g. NaCl, within the rangeof about 1 to 26% by weight of the anolyte are typical.

In general, the process of the present invention, whether using a brineor hydrochloric acid feed, may be operated over a wide temperaturerange, e.g. from room temperature up to the boiling point of theelectrolyte, although temperatures of from about 65° to 90° C arepreferred. Similarly, a wide variation in the electrical operatingconditions is also possible. Thus, for example, cell voltages of fromabout 2.3 to 5 volts and anode current densities of from about 0.5 to 4amps/in² are suitable.

The housing or outer casing member and cover of the electrolytic cell isformed of any electrolytically nonconductive material which is resistantto chlorine, hydrochloric acid and caustic alkali and which willwithstand the temperatures at which the cell may be operated. Generally,as has been indicated these temperatures are preferably from about 65 to90° centigrade. Exemplary of the materials which may be used are hightemperature polyvinyl chloride. hard rubber, chlorendic acid basedpolyester resins, and the like. It will be appreciated that thematerials of construction for this housing member preferably havesufficient rigidity as to be self-supporting. Alternatively, however,the housing may be formed of a material which does not fulfill all theabove mentioned criteria, such as concrete or cement, which materialsare not resistant to hydrochloric acid and chlorine, and have theinterior exposed areas of such members coated with a material which doesfulfill these requirements. Additionally, even in the case of materialswhich are substantially self-supporting, such as rigid polyvinylchloride, it is desirable on occasion such as in the instance ofrelatively large installations to provide reinforcing members around theexterior of the member, such as metal bands, to provide additionalrigidity.

The electrodes for the present electrolytic cell may be formed of anyelectrically conductive material which will resist the corrosive attackof the various cell reactants and products with which they may come incontact, such as alkali metal hydroxides, hydrochloric acid, andchlorine. Typically, the cathodes may be constructed of graphite, iron,steel, or the like, with steel being generally preferred unless stronghydrochloric acid solute is being electrolyzed. Similarly, the anodesmay be formed of graphite or may be metallic anodes. Typically, wheremetallic anodes are used, these may be formed of a so-called "valve"metal, such as titanium, tantalum or niobium as well as alloys of thesein which the valve metal constitutes at least about 90% of the alloy.The surface of the valve metal may be made active by means of a coatingof one or more noble metals, noble metal oxides, or mixtures of suchoxides, either alone or with oxides of the valve metal. The noble metalswhich may be used include ruthenium, rhodium, palladium, iridium, andplatinum. Particularly preferred metal anodes are those formed oftitanium and having a mixed titanium oxide and ruthenium oxide coatingon the surface, as is described in U.S. Pat. No. 3,632,498.Additionally, the valve metal substrate may be clad on a moreelectrically conductive metal core, such as aluminum, steel, copper, orthe like.

Instead of having a separate anode and cathode, i.e. monopolarelectrodes, if desired, the electrodes may be bipolar. In this instanceone side of the electrode will be positive and the other side negative.Although such a bipolar electrode may be homogeneous in composition,preferably it will be formed of steel on the cathode side and anactivated valve metal, such as titanium on the anode side, the titaniumbeing coated with one or more noble metals, noble metal oxides or thelike, as has been described above.

When sodium chloride solutions are electrolyzed in this cell employingthe sandwich barrier described, which is substantially impervious toliquids and gases and has a structure such that it operates as a solidionized salt, said structure being maintained rigid by the chargednetwork of negative ions or aggregates of negative ions electricallybalanced by a number of positive ions which are free to move in andthrough the structure, i.e., a cation active barrier, it is evident thatwhen the cathode compartment is initially charged with water or diluteaqueous sodium hydroxide, the anode compartment being charged withsodium chloride solution, chloride ions will be attracted to the anodeand discharged thereat. Sodium ions will pass through the barrierwhereas chloride ions and sodium chloride will be substantiallyprevented by the impervious barrier from entering into the cathodecompartment. Since essentially only sodium ions pass through the barrieran are discharged at the cathode, essentially salt free sodium hydroxideis produced in the cathode compartment. Similarly, when employing thecation active barrier in accordance with this invention, hydroxyl ionsare effectively prevented from migrating from the cathode compartmentthrough the barrier into the anode compartment. The current willtherefore be carried substantially exclusively by the sodium ions fromthe anode to the cathode and the difficulties caused by the backmigration of the hydroxyl ions are substantially eliminated by theprocess of this invention.

The new process results in the advantages of low voltage drop in thecell, production of highly pure, i.e., essentially salt free,concentrated solutions caustic soda, operation of the cell at relativelylow cell voltage, high current efficiency and, higher causticefficiency, than in similar cells wherein only a single layer of thepermselective membrane is used as the barrier separating the anode andcathode compartments. Moreover, because of the compatibility of thepermselective membrane in both chlorine and caustic alkalineenvironments at elevated temperatures, e.g., about 80° to 110°centigrade, the membranes can be maintained in continuous service forextended periods, surprisingly longer than the permselective membranesof the prior art process.

The barriers useful in the practice of the present invention canadvantageously be prepared and utilized in the form of multiple layersof thin films, either as such or deposited on as inert support, such asa cloth woven of Teflon® or glass fibers. The thickness of the supportedmembrane layers can be varied over a considerable range for example,from about 5 to 15 mils in thickness.

The barrier can be fabricated in any desired shape. As generallyprepared the co-polymer is obtained in the form of the sulfonylfluoride. In this non-acid form the polymer is relatively soft andpliable, and can be seam- or butt-welded forming welds which are asstrong as the membrane material itself. It is preferred that thepolymeric material be shaped and formed in the non-acid state. Followingshaping or forming into the desired membrane configuration, the materialis conditioned for use by hydrolyzing the sulfonyl fluoride groups tofree sulfonic acid or sodium sulfonate groups by boiling in water orcaustic alkaline solutions. On boiling in water for about 16 hours, theconditioned membrane material undergoes swelling, about 28 percent,which is isotropic, about 9 percent in each direction. When exposed tobrine, the swelling is reduced to about 22 percent which results in anet tightening of the membrane in use. The conditioning process can becarried out either out of the cell or with the barrier in place in thecell. Preferably the barrier is formed by placing two or more layers ofthe unconditioned membrane one upon the other in a suitable frame andthereafter inserting the unconditioned membrane sandwich barrier inplace in the cell and carrying out the conditioning process.

It has been found that the caustic efficiency of processes involving theuse of a permselective membrane material is increased by sandwichingtogether two or more membranes of this copolymeric material andinserting the sandwich as a barrier between the anode and cathode.

The sandwich can be composed of two or more layers of the permselectivemembrane material or alternate layers of the said membrane and porousmembranes, e.g., asbestos woven polypropylene cloth, woven Teflon® clothand the like. Thus the barrier can be a sandwich of two layers of theabove described homogeneous fluorocarbon copolymer, a sandwich of twolayers of said copolymer separated by a porous membrane of asbestossheet, a four layer sandwich of a first layer of asbestos sheeting, twolayers of the copolymer membrane and a final layer of asbestos sheeting.Other combinations of porous materials and permselective membranes willbe obvious to those skilled in this art. It will be apparant that as thethickness of the barrier is increased the voltage drop through thebarrier will increase and hence the power consumption factor and theresultant increase in cost will militate against the use of barriers ofexcessive thickness. It will thus be a matter of balancing the increasein caustic efficiency obtained in accordance with the invention againstthe increase in power costs which will determine the optimumconformation of the sandwich barrier.

It has been found further that whereas the caustic efficiency of a brineelectrolysis cell operated with a single layer permselective barriervaries from about 75% with a catholyte caustic concentration of about150 gpl to about 50% with caustic concentratons of about 260 gpl, whenthe known single layer barrier is replaced with a two layer sandwich ofthe permselective membrane, the caustic efficiency varies from about 86%at 150 gpl to about 63% at about 390 gpl.

It has thus been found that by the use of the sandwich barriers of thisinvention the caustic efficiency of the new processes is not onlysubstantially increased over that of similar processes wherein a singlemembrane barrier is used but also this caustic efficiency characteristicof the sandwich barrier decreases at a slower rate and remains at apractical value at higher catholyte caustic concentrations.

The following examples illustrate certain preferred embodiments of thepresent invention. Parts and percentages are by weight and temperaturesare given in degrees centigrade unless otherwise indicated.

EXAMPLE 1

A solution of sodium chloride containing about 250 gpl of sodiumchloride and sufficient hydrochloric acid to maintain the anolyte pHwithin the range of about 3.0 to 4.5 was continuously introduced intothe anode compartment of a two compartment electrolytic cell asillustrated in the drawing and containing a ruthenium oxide coatedtitantium mesh anode and a steel mesh cathode. The electrodes, of 60 sq.in. effective area, were separated by two layer sandwich of the cationactive permselective membrane as a barrier. The membrane sandwich has aneffective area of 60 sq. in. and was composed of two layers of a 10 milthick film of a hydrolyzed copolymer of tetrafluoroethylene andsulfonated perfluorovinyl ether of equivalent weight of about 1100,prepared according to U.S. Pat. No. 3,282,875, and conditioned to thefree acid form by soaking in boiling water for about 16 hours.

The catholyte compartment was fed continuously with water, whichtogether with the water which passed through the barrier by osmosis,maintained the catholyte liquid level constant. Caustic liquor producedin the catholyte compartment flowed from the cell through the overflowpipe continuously and was collected for a period of about 16 hours, andsewered for about 8 hours. The temperature was varied between about 60°and 85°, each variation being maintained for about 24 hours. The cellwas operated continuously over a period of 24 days using a currentdensity of one ampere per square inch of diaphragm. The data collectedin this experiment is set out in the following table.

                  TABLE                                                           ______________________________________                                                                     NaC1 in       NaOH                                                            Catho-  Avg.  Eff-                                    Temp.   Anolyte         lyte    NaOH  ciency                             Day  °  C                                                                           pH       Voltage                                                                              gpl     gpl   %                                  ______________________________________                                         4   60      3.9      3.63   1.16    290   72.2                                5   65      3.9      3.63   0.82    256   82.0                                6   67      4.0      3.54   0.44    137   86.3                                8   59      3.1      4.11   0.47    318   70.3                                9   61      3.4      4.12   0.19    230   86.4                               10   81      3.9      3.22   0.37    186   83.4                               12   82      3.7      3.45   0.33    383   62.8                               14   82      2.2      3.53   0.35    355   74.1                               15   78      4.3      3.20   0.58    170   85.9                               16   82      4.3      3.08   0.26    153   86.1                               17   79      4.2      3.29   0.30    128   81.4                               20   75      4.4      3.31   0.40    295   67.3                               23   76      4.4      3.30   0.54    349   66.7                               ______________________________________                                    

These data indicate the consistently high caustic effciency obtainedusing the sandwich type barrier of this invention. These efficienciesare about 10 to 15% higher than those obtained under similar conditionsusing a single membrane barrier at relatively low caustic concentrations(about 150 gpl) and about 20 to about 30% higher at higher causticconcentrations (about 300 gpl and higher).

EXAMPLE 2

In this Example, aqueous sodium chloride was electrolyzed in aconventional two compartment cell equipped with a bipolar electrodehaving a steel cathode face and a ruthenium oxide coated titanium cladsteel mesh anode face. Three such two compartment cells were enclosed ina single housing. The electrodes of the first two cells ("A" and "B")were separated by a barrier composed of a single 7 mil thickpermselective membrane of a hydrolyzed copolymer of tetrafluoroethyleneand sulfonated perfluorovinylether of equivalent weight of about 1100.The third cell contained a barrier composed of two layers of the samecopolymer membrane. The electrodes were of 60 sq. in. effective area anda current of 60 amperes was expressed across the electrodes.

At the start of the run, aqueous caustic soda containing 50 gpl NaCH wascirculated through the cathode compartments and acidified aqueous sodiumchloride containing about 250 gpl NaCl and acidified to a pH of about3.5. The cell was operated at about 65° for 4.5 hours, collecting thecaustic liquor produced.

After this period of continuous operation in cell A, the cell voltagewas 3.08 volts, the catholyte liquor contained 333 gpl NaOH, whichresults in a caustic efficiency of 49.1%.

In cell B, the cell voltage was 3.25 volts, the catholyte liquorcontained 381 gpl NaOH and thus its caustic efficiency was 59.5%.

In cell C, the cell voltage was 3.65 volts, the catholyte liquorcontained 392 gpl NaOH, and its caustic efficiency was 72.2%.

These data indicate the improved caustic efficiency obtained when usinga sandwich type barrier of this invention in a bipolar electrode typecell.

It is to be understood that the foregoing specification and examplesdescribe the invention by reference to certain embodiments thereof. Manyvariations in the details given above will be apparent to those skilledin the art and such variations which do not depart from the scope orspirit of the invention are intended to be covered herein.

What is claimed is:
 1. An electrolysis cell comprising a housing, ananode, a cathode, and a permselective barrier substantially imperviousto liquids and gases separating said anode and said cathode, saidbarrier consisting essentially of at least two layers of a hydrolyzedcopolymer of tetrafluoroethylene and a sulfonated perfluorovinyl etherof the formula

    FSO.sub.2 CF.sub.2 CF.sub.2 OCF(CF.sub.3)CF.sub.2 OCF═CF.sub.2

said copolymer having an equivalent weight of from about 900 to
 1600. 2.A cell as described in claim 1 wherein said copolymer has an equivalentweight of from about 1100 to
 1400. 3. A cell as described in claim 1wherein said barrier consists of two layers of said hydrolyzedcopolymer.
 4. A cell as described in claim 3 wherein each layer of saidbarrier is of a thickness within the range of from about 5 to about 15mils.
 5. A cell as described in claim 1 wherein said barrier consistsessentially of a sandwich of at least two layers of said copolymerseparated by a porous membrane.
 6. A cell as described in claim 5wherein said porous membrane is composed of asbestos sheet.
 7. A cell asdescribed in claim 6 wherein the sandwich barrier consists of two layersof said copolymer separated by a porous membrane of asbestos sheet.
 8. Acell as described in claim 1 wherein said anode is a metallic anode. 9.A cell as described in claim 8 wherein said metallic anode consistsessentially of a valve metal, the surface of which is coated with acoating comprising a noble metal or oxide thereof.
 10. A cell asdescribed in claim 9 wherein said valve metal is titanium.
 11. A cell asdescribed in claim 10 wherein the titanium metal is coated with acoating comprising a mixture of ruthenium oxide and titanium oxide. 12.A cell as described in claim 11 wherein said metal anode is formed of atitanium substrate on an aluminum core, said substrate being coated witha coating comprising a mixture oxide and titanium oxide.
 13. Anelectrolysis cell comprising an anode compartment, a cathodecompartment, at least one bipolar electrode and a permselective barriersubstantially impervious to liquids and gases separating the anodecompartment from the cathode compartment of said cell, said barrierconsisting essentially of at least two layers of a hydrolyzed co-polymerof tetrafluoroethylene and a sulfonated perfluorovinyl ether of theformula

    FSO.sub.2 CF.sub.2 CF.sub.2 OCF(CF.sub.3)CF.sub.2 OCF═CF.sub.2

said co-polymer having an equivalent weight of from about 900 to about1600.
 14. A cell as described in claim 13 wherein the cathode side ofsaid bipolar electrode is formed of steel and the anode side of saidbipolar electrode is formed of a valve metal having a coating thereoncomprising a noble metal or noble metal oxide.
 15. A cell as describedin claim 14 wherein said valve metal is titanium having a coatingthereon comprising ruthenium oxide.